World: r3wp

DO is like REDUCE, in that it evaluates every item in the block, 
except DO does not create and store results in a new block - it just 
returns the last value.

yes Kib, blocks are mainly used to pass several value

understood, now the question is what we cannot do in Rebol ?!

You can't really say that in rebol everything is a block, but you 
can say everything is *in* a block. Most rebol code is found in blocks. 
Blocks rule as the container of choice in the rebol universe!

What we cannot do in rebol:

i will say, what we cannot do without pain. All is possible but some 
are painfull

- Create references to parts of datatypes. Eg. Make a variable which 
aliases the y component of a pair!

Hum, we can with tiny functions accessors, the drawback is that it's 
more slow than a real reference

A pair! is a datatype which looks like a series, but is unfortunately 
(for this case) a scalar. That means any modification causes the 
whole pair to be copied.

I think I've found a good exercice...if only the following is true 
: Rebol seems to handle prefix notation

Operators can be prefix or infix

>> 2 + 3
== 5
>> + 2 3
== 5

Steeve, yes, you can do that, but then if other people want to use 
your code, they must use the accessors you have written, which effectively 
expands the language.

really strange : "+ 2 3" returns 5, but "- 2 3" returns 3

The - there is actually unary.

Yes, strange!

A negate.

ah yes. :-D

lol

It's the same as typing only  "2 3"

The 2 is evaluated (to 2), then the 3 is evaluated and returned, 
because it's last.

-2 3
 instead

Anton: that's it. So then  "+ 2 -3"

(Steeve, yes, more similar.)

- is the (only?) operator which cannot be used infix.

Sorry, that's an incorrect statement.

To get list of operators:

>> ? op!

Kib but for our sake, there is an infix version of - operator: subtract

You mean prefix.

- cannot be used as binary prefix operator, unlike the others + * 
/

Yes.

Steeve : thanks, i was just lookin for something like that

i don't like this name by the way, Carl should have renamed it: sub, 
since long time ago

I never use SUBTRACT because it's too long ! :-)  I always just use 
parens to allow me to use -, even if evaluation is slower.

Steeve : +1

Using parens, it's more clear this way.

sub suuuuuuuuuuuuuuuub ;-)

like in rebcode

Most operators have prefix only twins (that are called actions). 
Like: equals? is the twin of =

equal?

(without s)

multiply, add, subtract, divide, remainder, lesser?, lesser-or-equal?, 
not-equal?, equal?, strict-equal?, same?, greater?, greater-or-equal?

Geomol: power

beware with same? it's not really a math operator

If you prefer SUB instead of SUBTRACT, just do this in the beginning:

sub: :subtract

ah yes, POWER aslo.

*also*

I think, SAME? is like =?

yes but it's used for serie manipu

or objects, not for maths

>> [a b c] = [a b c]
== true
>> [a b c] == [a b c]
== true
>> [a b c] =? [a b c]
== false